This invention relates to polyphenylene ether (PPE) blends having improved processability and reduced adhesion is metal surfaces, and more particularly to a method for improving the processibility of PPE blend compositions.
Polyphenylene ether (PPE) resins and particularly PPO, or poly(2,6-dimethyl-1,4-phenylene ether), have long been known as high temperature thermoplastics. These resins are typically prepared by the oxidative coupling of a monohydric phenol such as 2,6-dimethyl phenol, as taught for example in U.S. Pat. Nos. 3,134,753 and 3,306,874. PPE resins soften or melt only at very high temperatures, normally well above 500.degree. F., which may exceed the upper limit of thermal stability for such materials. The resins also exhibit a very high melt viscosity, and are very difficult to melt process in conventional processing equipment without substantial thermal degradation. Commonly, the resins are blended with lower melt temperature resins to improve their melt processability and to achieve commercially acceptable molding characteristics. For example, polyphenylene ether resins may be blended with styrenic resins as is disclosed in U.S. Pat. Nos. 3,356,761 and 3,383,435 or with polyolefins such as polyethylene as described in U.S. Pat. No. 3,361,851 to achieve improved processabiity.
More recently, in U.S. Pat. No. 4,038,543, there were disclosed phenylene ether copolymer resins (PEC) having improved thermal resistance, high temperature oxidative stability and resistance to heat aging. These resins, copolymers of 2,6-dialkyl phenols and 2,3,6-trialkyl phenols containing up to 50 wt% of the trialkyl monomer component, are more stable at the melt processing temperature, but exhibit a high melt viscosity and are also difficult to melt process alone. Blending these copolymers with other resins such as styrenic polymers is therefore desirable to achieve adequate processability and a useful balance of mechanical properties for commercial molding applications.
Although combining polyphenylene ether resins or phenylene ether copolymer resins with styrenic resins achieves an improvement in processability and when further formulated produces molding resins with a useful balance of mechanical properties, such blends generally require the further addition of processing aids to achieve practical thermal processability characteristics in molding and extrusion equipment. For example, polyolefins have long been added to such compositions as processing aids, and the use of hydrocarbon waxes to improve melt flow is disclosed in U.S. Pat. No. 4,410,654. The mold release characteristics of such compositions are also said to be good when a minor amount of a low molecular weight polyamide or alkylene glycol is substituted for the polyolefin, according to U.S. Pat. No. 4,433,088.
The use of polyolefins, hydrocarbon waxes, amides and other such processing aids has been successful in providing processable blends. However, when used at levels sufficient to improve processing without significantly lowering the heat distortion temperature characteristics of these blends, the resulting formulations exhibit an undesirable degree of adhesion to hot metal surfaces and sticking to the surfaces of dies, molds and the like. A reduction in mold-sticking characteristics has heretofore been accomplished by further increasing the level of the processing aid, thereby reducing the heat distortion characteristics of the blend.